1. Field of the Invention
This invention relates to pneumatic redial tires, and more particularly to a heavy duty pneumatic radial tire used from small-size truck to middle- to large-size vehicles such as trucks, buses and the like which improves a belt durability when being run on good roads exemplified by paved road surface or the like at a relatively higher speed and is excellent in the resistance to tire damage based on exterior input such as cut burst, shock burst or the like which is liable to be caused when being run on bad roads exemplified by non-paved road surface.
2. Description of Related Art
Recently, high-speed running and diversification for running mode progress with the advance of improving the performances of vehicles, while the lowering of deck height in the vehicles is generalized. Therefore, the service conditions for pneumatic tires used in these vehicles, particularly pneumatic radial tires used in vehicles of from small-size truck to truck and bus tend to become more severer as compared with the conventional service conditions. On the other hand, the demand for recapped tires is increasing in the middle- to large-size vehicles, and particularly tires capable of conducting the recapping several times is demanded.
Under the above situations, since belt durability in the pneumatic radial tire is particularly an important performance, not only the occurrence of separation failure from a belt end in a new tire but also the occurrence of large cracks in the belt end of a used base tire to be recapped becomes a problem. Such a lacking of belt durability is particularly serious in low-section profile tires used in low deck vehicles.
Among vehicles such as small-size truck, usual-size truck and the like, there is a dump truck mainly running on non-paved road. Further, sightseeing buses and so on are obliged to sometimes run on non-paved road apart from city buses. Although it is unavoidable that protuberant pebble stones and quarrying are scattered on the non-paved road, foreign matters such as metallic pieces having a sharp edge may be existent even on a paved road surface. Therefore, when the tire mounted on these vehicles rides on the foreign matter such as protuberant pebble stones, quarrying, metallic piece and the like, there may be caused cut damage arriving at the belt or burst failure based on the cut damage, or there may be caused shock burst prior to the cut damage.
It need scarcely be said that the structure of the belt is deeply concerned in the aforementioned problems on the belt durability and on the cut burst or shock burst. In general, the conventional belt has a structure shown by symbol 6A in FIG. 10. The belt 6A is comprised of four steel cord layers 6A-1 to 6A-4, in which at least two adjacent layers are cross cord layers, cords of which layers being crossed with each other with respect to an equatorial plane E of the tire.
In the belt 6A of the conventional tire, an inclination angle of steel cord with respect to the equatorial plane E is 50-70xc2x0 in an innermost layer 6A-1 and 15-25xc2x0 in middle layers 6A-2, 6A-3, and that of an outermost layer 6A-4 is the same as in the middle layer 6A-3. The middle layers 6A-2, 6A-3 are cross cord layers, and the outermost layer 6A-4 has the same cord inclining direction as the middle layer 6A-3 and serves as a protection layer to external wound. In the tire having such a belt 6A, however, interlaminar shearing strain in end portions of the middle layers 6A-2 and 6A-3 having a large cord crossing angle is remarkably large during running under loading and interlaminar separation failure based thereon is liable to be caused and also durability to heat generation is lacking, so that the outermost layer 6A-4 does not sufficiently play a role as the protection layer. Also, this tire has a problem that cut burst failure or shock burst failure is substantially defenseless. These problems are serious in low-section profile tires having a small aspect ratio.
In order to solve the above problems, JP-A-57-201704 proposes a heavy duty pneumatic radial tire used under a higher internal pressure in which at least two circumferential cord layers are disposed on an outer circumference of a carcass and two or more cross cord layers are disposed on a circumference thereof, and cords of the circumferential cord layer have a relative elongation before fracture of at least 8% and a heat shrinkage of at least 1.25% prior to the inflation of air pressure, and cords of each of the cross cord layers have an inclination angle of 45-90xc2x0 with respect to the equatorial plane of the tire and are extensible and heat shrinkable, and JP-A-2-81706 proposes a heavy duty pneumatic tire in which a belt is comprised of a first strip disposed on an outer circumference of a carcass and containing wavy or zigzag reinforcing elements arranged along the equatorial plane, and a second strip of at least two layers, cords of which layers being crossed with each other at an inclination angle of 30-60xc2x0 with respect to the equatorial plane.
However, the tires disclosed in these publications generate a large shearing strain in end portions of the cross cord layers during the running under loading because the belt is provided with the adjoining laminated cross cord layers, cords of which layers being crossed with each other with respect to the equatorial plane. As a result, the interlaminar separation failure is liable to be caused in the end portions of the cross cord layers, so that the belt structure disclosed in these publications is naturally critical in the improvement of the belt durability and hence the anticipated belt durability can not be obtained. In the tire of JP-A-57-201704, organic fiber cords are used as a reinforcing element for the belt, so that there is also a problem that the resistance to cut burst or shock burst is lacking.
In relation to the problem on the separation at the end portions of the cross cord layers as mentioned above, JP-A-2-81705 proposes a belt structure comprised of a first strip containing wavy or zigzag reinforcing elements arranged along the equatorial plane of the tire, and a second strip of a single layer containing cords arranged at an inclination angle of 15-70xc2x0 with respect to the equatorial plane. JP-A-8-318706 proposes a belt structure comprised of a single slant belt layer disposed on an outer circumference of a carcass and containing a plurality of cords or filaments inclined with respect to the equatorial plane, and at least one circumferential belt layer disposed at an outside of the slant belt layer and containing a plurality of organic fiber cords arranged in parallel to the equatorial plane.
The tires having the belt structure disclosed in the latter two publications are concerned with a pneumatic radial tire for passenger car and are designed to establish the reduction of weight and cost and the improvement of performances for passenger tire such as cornering property, durability, high-speed durability and the like. They do not have cross cord layers, so that there is not an anxiety of causing the separation at the end portions of the cross cord layers. However, they are not used at a considerably high air pressure under a large load such as heavy duty pneumatic radial tire of the invention here, so that they have no concern with the improvement of performances in the heavy duty pneumatic radial tire such as belt durability, resistance to cut burst, resistance to shock burst and the like.
It is, therefore, an object of the invention to provide heavy duty pneumatic tires having mainly an aspect ratio of not more than 75% which have a sufficient resistance to belt separation in a new tire and improve a durability of belt end, which is unacceptable in repetitive recapping after use, to provide a belt durability sufficiently durable to reuse as a recap tire, and have excellent resistances to cut burst and shock burst and are capable of largely prolonging total service life of the tire.
According to the invention, there is the provision of a pneumatic radial tire comprising a pair of bead portions, a pair of sidewall portions, a tread portion extending between both sidewall portions, a carcass of one or more rubberized cord plies of radial cord arrangement extending between a pair of bead cores embedded in the bead portions to reinforce the bead portions, sidewall portions and tread portion, and a belt reinforcing the tread portion on an outer circumference of the carcass and comprised of at least two rubberized steel wire element layers, in which an outermost belt layer is an outermost slant element layer containing the steel wire elements obliquely arranged with respect to an equatorial plane of the tire; at least one belt layer of the belt located inward from the outermost slant element layer in a radial direction of the tire is a circumferential element layer containing the steel wire elements arranged substantially in parallel to the equatorial plane; a coating rubber for the steel wire element in the outermost slant element layer has a modulus of elasticity at compression of not less than 200 kgf/cm2; and a sum of the cross sectional area of the steel wire elements in the circumferential element layer per unit length in a direction of extending the steel wire element in each of the belt layers is larger than a sum of the cross sectional area of the steel wire elements in the outermost slant element layer per the same unit length.
In each belt layer of the belt, many steel wire elements are embedded in rubber. The term xe2x80x9csteel wire elementxe2x80x9d used herein includes a steel cord obtained by twisting plural steel filaments and a bundle of one or more steel filaments without twisting.
The modulus of elasticity at compression is calculated from a load L and a displacement when a vulcanized rubber piece 21 to be tested is fully filled in a steel jig 20 having a right cylindrical cavity of 14 mm in inner diameter d and 28 mm in height h as shown in FIGS. 6 and 7 and the jig 20 is set on a compression testing device 22 and a load L is applied onto upper and lower faces of the vulcanized rubber piece 21 at a rate of 0.6 mm/min, during which a displacement of the vulcanized rubber piece 21 is measured by means of a laser displacement meter 23, and is a value at a compression ratio of 0.5%. Moreover, a testing temperature is 25xc2x0 C.
The term xe2x80x9csum of the cross sectional area of steel wire elements in a direction of extending the steel wire element in each belt layerxe2x80x9d used herein means a sum of the sectional area of all steel wire elements included in a unit length, e.g. 25 mm or 50 mm as measured along the belt layer in a direction perpendicular to the direction of extending the steel wire element in the belt layer, in which the outermost slant element layer indicates the sum of one layer and the circumferential element layer indicates the sum of all layers. The measuring position is not specified, but it is a central region of the tread portion including at least an equatorial plane of the tire.
As to the sum of sectional area of steel wire elements, it is preferable that the sum of sectional area of the steel wire elements in the circumferential element layer is not less than 2 times the sum of sectional area of the steel wire elements in the outermost slant element layer.
In a preferable embodiment of the invention, the steel wire element in the circumferential element layer has an initial elongation within a range of 1-5% prior to vulcanization and indicates an initial elongation within a range of 0.2-3.0% in a product tire after vulcanization. The term xe2x80x9cinitial elongationxe2x80x9d used herein is defined by an elongation (%) just before a gradient of a curve rapidly increases in load (horizontal axis)xe2x80x94elongation (vertical axis) curve of the steel wire element. And also, the initial elongation prior to vulcanization means an elongation of the steel wire element from raw material stage just before the completion of crosslinking reaction at vulcanization, and the initial elongation in the product tire means an elongation of the rubberized steel wire element taken out from the tire.
In another preferable embodiment of the invention, the circumferential element layer is comprised of steel wire elements arranged in form of a wave having an amplitude in a widthwise direction of the belt. The wave includes sinusoid, zigzag and the like and is maintained in the tire irrespectively of the wavy form.
In the other preferable embodiment of the invention, the steel wire element in the outermost slant element layer has an inclination angle of 35-55xc2x0 with respect to the equatorial plane of the tire.
In the further embodiment of the invention, an inner slant element layer containing steel wire elements obliquely arranged with respect to the equatorial plane is disposed between the carcass and the innermost layer among the circumferential element layers.
The inner slant element layer is favorable to have a width exceeding a width of the circumferential element layer and also the steel wire element in the inner slant element layer is favorable to have an inclination angle of 10-70xc2x0 with respect to the equatorial plane.
In view of the separation resistance, it is preferable that the steel wire element of the outermost slant element layer and the steel wire element of the inner slant element layer are crossed with each other with respect to the equatorial plane of the tire.
In the other preferable embodiment of the invention, an end portion of the inner slant element layer is folded so as to enclose all of the circumferential element layers from one of widthwise ends thereof toward the other widthwise ends to form a folded layer portion serving as the outermost slant element layer, or both end portions of the inner slant element layer are folded so as to envelop all of the circumferential element layers from both widthwise ends thereof toward the equatorial plane to form a folded layer portion serving as the outermost slant element layer.